m_mag2geo & m_geo2mag complain of "Attempt to reference field of non-structure array"

Hi, This problem persists regardless of the arguments i enter (i.e. which lat and long). The function calls the private m_map function

 mc_coords

which hits the error on line 74:

 lambda=MAP_PROJECTION.coordsystem.lambda;

Anyone else experience this? Any ideas?

Cheers,

Neil

Answer 1

[Geoff Hayes]

Neil - this error message is telling you that the code is treating a variable as if it were a structure with fields when in fact (for whatever reason) it isn't. For example, the following code generates the same error

 x = [];
 x.test;

 Attempt to reference field of non-structure array.

The function in question, mc_coords is assuming that MAP_PROJECTION is a structure with a field named coordsystem. If you look at the code from mc_coords.m, this variable is global

 global MAP_PROJECTION MAP_COORDS

If this variable has not been initialized elsewhere then it will be empty (like x in the above example) and so will throw the error. What you need to do is run the m_proj function which will initialize the map projection global variable (and others as well). You will be required to choose the projection type and provide any supporting information.

For example, to use the orthographic projection, type

 m_proj('set','Orthographic')

to see what attributes/parameters can be set for this projection. To choose this projection with default values use

 m_proj('Orthographic')

To view all projections, type

 m_proj('set')

[Neil]

Thanks a lot for this answer Geoff. I'm happy to have overcome this problem, but I think the m_geo2mag functions could explicitly state that they require other input to work. I'm not sure why it's neccesary to specify a projection as, in using this function, one is not actually mapping anything but rather changing between two (spherical) bases. What difference could different projections make on the outcome? Magnetic declination shouldn't depend on such a choice.

[Geoff Hayes]

Neil - those are good questions and should be directed to the author of the code.